It is common practice to inductively heat a cylinder or tube of a ferromagnetic (high magnetic permeability) material, such as steel, by an induction (eddy) current. The eddy current is induced in the ferromagnetic material by an applied magnetic flux, and the magnetic flux is generated by passage of an alternating current through one or more heater coils disposed around the cylinder or tube. This method of inductive heating can be adapted to various other types of materials, work pieces and loads, including fluid, semisolid or solid materials (e.g., molten steel or magnesium filled and non-filled polymers, billets and ceramics).
The article to be heated may itself be heated by an induction current, and/or it may be in thermal communication (e.g., by conduction or radiation) with another article or substrate being inductively heated, for example, when heat inductively generated in a ferromagnetic substrate is transferred to a semiconductor wafer. In this regard, the electrical resistivity of the heating element or coil may be varied, for example using a more resistive material to increase the amount of resistive heat generated in the coil and transferred to the article (by conduction or radiation). Nichrome, a nickel chromium (NiCr) alloy having about sixty times the electrical resistivity of copper, has been used for the coil to generate both a magnetic flux for inductive heating of an article lying within the flux, and resistive heat (in the coil), which is then transferred by conduction and/or radiation to the same article.
Traditional inductive heating coils are made of copper and are water cooled to prevent overheating of the coil. Also, an air gap is provided between the water-cooled coil and the article being heated, to avoid removal of heat from the article by the coil cooling medium. The air gap and cooling requirements increase the complexity and cost of the heating system. They also reduce the strength (structural integrity) of the apparatus, which can be critical in applications where pressure is applied, e.g., a compression mold. However, without cooling, the coil is subject to failure (melting or burn out at elevated temperatures). Traditional inductive heating systems do not utilize more highly resistive (e.g., NiCr) coils, because the enhanced resistive heating of the coil would make coil cooling even more difficult, requiring still larger cooling channels and/or lower cooling temperatures, each of which results in greater energy consumption and cost. Furthermore, a resistive load cannot be driven by a traditional inductive power supply.
There is an ongoing need for heating systems and methods which address some or all of these problems and/or for energy sources to power such heating systems more efficiently and preferably at a lower cost.